This section provides background information related to the present disclosure which is not necessarily prior art.
Transmissions that have planetary gear sets typically include a sun gear, a ring gear, a planet carrier, and a plurality of planet gears. The planet gears are mounted to the planet carrier for common rotation about a central axis of the sun and ring gears, while being rotatable relative to the planet carrier. The planet gears are meshingly engaged with both the ring gear and the sun gear. One of the sun gear, ring gear, or planet carrier is typically non-rotatably coupled to a housing of the transmission and the other two of sun gear, ring gear, or planet carrier receive and/or transmit torque between input and output members of the transmission.
The geometry of the gear teeth, tolerances, and/or wear of the gear teeth can result in backlash or play between the sun gear, ring gear, and planet gears. Backlash between the gears of the planetary gear set can result in undesirable noise, vibration, and/or harshness (“NVH”) during load cycling of the transmission or when reversing the rotational direction of the input member, as the backlash must be taken up before torque can be transferred between the gears of the planetary gear set.
In vehicles which employ an electric motor for propulsive power or for torque vectoring, the electric motor typically is drivingly coupled to a transmission including a planetary gear set to provide torque thereto on an as-needed basis. The electric motor can be operated in an intermittent fashion and can frequently change rotational direction (e.g. based on tractive need and/or whether the vehicle is making a right or left turn), which can result in undesirable NVH due to backlash in the planetary gear set.
Furthermore, planetary gear sets can suffer from torque sharing, or unequal loading of the planet gears, especially when the planetary gear set includes four or more planet gears. Typically, such unequal load sharing is addressed by increasing the size of the planet gears, which can result in undesirable increases in size, weight, and cost of the transmission.